Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP33

HIGH-POWER TESTS OF REPAIRED CIRCULATOR FOR LIPAc RFQ
K. Hirosawa†, N. Kubo, A. De Franco, K. Kondo, K. Masuda, A. Kasugai, M. Sugimoto, Quantum
National Institutes for Quantum Science and Technology (QST), Aomori, Japan
I. Moya, F. Scantamburlo, Y. Carin, Fusion for Energy (F4E), Garching, Germany
L. González-Gallego Sánchez-Camacho, C. Caballero, J. C. Morales Vega, University of Granada(UGR), Granada, SpainAbstract
The Linear IFMIF Prototype Accelerator is the accelerator facility to validate the engineering design of the International Fusion Materials Irradiation Facility (IFMIF). The
first stage of the IFMIF accelerator up to 9 MeV for
125 mA deuteron beam in continuous wave (CW) mode is
the design configuration of the LIPAc. The 9.8 m long RFQ,
which is one of the most important accelerating components despite extremely strong space charge force, is fed by
8 RF drive sources. They are synchronized with the White
Rabbit based solution and each include 200 kW amplifiersystem including their circulator. During the RFQ CW
commissioning campaign in January 2022, one circulator
had frequently arc trouble leading to severe damages at theconnection of the inner transition from the coaxial line output of the final tetrode to the strip-line central plate of thecirculator. The damaged circulator was repaired at the manufacturer’s factory premises and tests of RF characteristics
were performed after its reinstallation in LIPAc. An overview of the encountered issues and repair, evaluating theparameters, and the points we found in the high-power test.

spent to recover the system over one year. The beam commissioning has been resumed from July of 2023.
The topic summarized in this report consists of the failure and repair of the circulator, evaluating parameters tosee the effect of temperature and intensity of magnetic field,
and the high-power loading test of full RF power operation.

RF SYSTEM
Overview of the LIPAc RF System
The LIPAc RF system consists of the source and its amplifiers for 1-RFQ, 2-Re-Bunchers, and 8-SRF cavities.
The control system is based on one typical design [1],shown in Fig. 1, used in the Re-buncher cavity. In addition
to the base model, Master-Slave control loop is implemented but tuner is removed for the RFQ, because the resonance tuning is realized by cooling water loops.

INTRODUCTION
LIPAc（The Linear IFMIF Prototype Accelerator）isthe accelerator to validate lower energy part of the designof IFMIF, which is the neutron source for the test facilityof the materials for the fusion reactor. In the design, thesystem is aiming to accelerate the 125 mA-CW deuteronbeam up to 9 MeV. We are now in the stage of intermediateof the full commissioning, so-called Phase-B+, which ispositioned between the Phase-B, RFQ acceleration, and thePhase-C, superconducting RF cavity and the high-power
beam dump.
We had a beam commissioning to validate the system
RFQ acceleration with the low energy beam dump, 125 mA
1 ms pulse is successfully accelerated in 2019. In present,
the accelerator construction has been done, except for thecryomodule linac, which can accelerate the beam from
5 MeV to 9 MeV, and the beam operation was started inJune of 2021. In parallel of the beam, RFQ commissioning
to be reached to CW, but then the destructive sparks are
happened in the transition part of a circulator, used to circulate the reflection power. In addition, during the optional
challenge to drive RFQ with 7 RF stations, vacuum leakfrom RF power couplers was happened. Because of above
two points in the R F system, a long maintenance term was

Figure 1: Schematic of the RF control loop.
The amplification system is also same for the RFQ and
the SRF cavities. Based on the tetrode RF power amplification and the circulator works to isolate the external impedance between the amplifiers and RF cavities [2-5]. The
difference between these two type of cavities is just component sizes coming from the estimation of the possible
ohmic loss in the circulator and the coaxial lines. The RFQand SRF system are required to provide 250 kW-CW and
150 kW-CW power, respectively. The control system isdriven with the very precise clock reference system basedon the White Rabit synchronization protocol, which has
sub-nano second scale precision.

_______________________________________________† hirosawa.koki@qst.go.jp- 923 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP33

Figure 2: Illustration of 1/8 RFQ-RF amplification system.

The RFQ-RF System of LIPAc
The RFQ system of our LIPAc is working as one 9.8 m
long resonator. To input 1.2 MW power including beamloading compensation, 8-synchronized RF stations areadapted as shown in Fig. 2 for one station. The reference
position of the calibration is just before the RFQ-RF coupler, and RF feed power is tuned to be balanced for RFQ
coupling level. This method can work on the assumption ofunchanged RF-coupler conditions and the uniformed circulator conditions in any power, phase, and pulse combination. Studies about the condition and the dynamics when
above assumption is broken are one of most important
point in our having RF system. First point of our starting
point related with the RF-coupler issue is discussing in thedifferent report in the meeting. The later topic, the issue ofcirculator, is dealt with in this report.
The circulator is used to isolate the RF transmission line
between tetrode system and the RF cavity to keep the coupling impedance of the tetrode cavity. Especially for theMaster-Slave control, the uniformity of the circulator characteristics is also considerable important point to be optimized. The “Master” station is driven by individual modewith clock synchronization. Controlled reference amplitude and phase for the RFQ field including the pulse timing
is provided from the master module, and the compensation
loop of the relative offset between the master and eachslave channel is applied at the point of before RF-couplers
for the forward power. Because the heat load compensationof the circulator is done by active tuning of the coil current,
if the characteristics of the S-parameters and the thermal
effect are different among the circular of each chain, allcalibrated power will be unbalanced. Circulator dynamics
is closely connected with the RFQ study in the control
point of view.

POWER CIRCULATION IN THE CIRCULATOR

Figure 3: Minimum setup to circulate RF for 3-ports.
The circulator has been studied by the methodologicalapproaches for a long time [6-8]. Figure 3 shows that theminimum setup of the circulator. Most of dynamics of thecirculation of electromagnetic waves is determined by theresonance characteristics of the soft ferrite disk, sandwiches the stripline of the large inner conductor plate. Dynamics in the ferrite is represented by the relative permeability [8], written in Eq. (1) and (2). Rectangle brackets isrepresenting matrix form.

𝝁𝝁[𝝁𝝁𝒓𝒓 ] = �𝒋𝒋𝒋𝒋
𝟎𝟎

−𝒋𝒋𝒋𝒋
𝝁𝝁𝟎𝟎

[𝝁𝝁𝒓𝒓 ] = [𝑰𝑰] + [𝝌𝝌],𝝌𝝌𝒙𝒙𝒙𝒙𝟎𝟎𝟎𝟎� , [𝝌𝝌] = �𝝌𝝌𝒚𝒚𝒚𝒚
𝟏𝟏𝟎𝟎

(1)
𝝌𝝌𝒙𝒙𝒙𝒙𝝌𝝌𝒚𝒚𝒚𝒚𝟎𝟎𝟎𝟎𝟎𝟎�. (2)𝟎𝟎

The susceptibility factors 𝝌𝝌𝒊𝒊𝒊𝒊 appeared in here can be determined by the radian frequency of the alternating radio
magnetic field 𝝎𝝎𝒎𝒎/𝒊𝒊 in the ferrite.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP33

��𝝌𝝌𝒙𝒙𝒙𝒙 = 𝝌𝝌𝒚𝒚𝒚𝒚 =−𝝌𝝌𝒚𝒚𝒚𝒚 = 𝝌𝝌𝒙𝒙𝒙𝒙 =𝝎𝝎𝒎𝒎 𝝎𝝎𝒊𝒊−𝝎𝝎𝟐𝟐 +𝝎𝝎𝟐𝟐𝒊𝒊𝒋𝒋𝝎𝝎𝒎𝒎 𝝎𝝎𝒊𝒊,(3)

had a 200 kW transmission test after on-site repair, and it
was cleared also for the long-term stability check.

−𝝎𝝎𝟐𝟐 +𝝎𝝎𝟐𝟐𝒊𝒊

𝝎𝝎𝒎𝒎 = 𝜸𝜸𝑴𝑴𝟎𝟎.
𝝎𝝎𝒊𝒊 = 𝜸𝜸(𝑯𝑯𝟎𝟎 − 𝑵𝑵𝒛𝒛 𝑴𝑴𝟎𝟎 )(4)where 𝑴𝑴𝟎𝟎 is the saturation magnetization [A/m], 𝜸𝜸 =𝟐𝟐. 𝟐𝟐𝟐𝟐 × 𝟏𝟏𝟏𝟏𝟓𝟓 is the gyromagnetic ratio [[rad/s]/[A/m]], 𝑯𝑯𝟎𝟎
is the applied direct magnetic field intensity [A/m], and 𝑵𝑵𝒛𝒛
is a z-directed shape demagnetizing factor of the geometry.
The mismatch or peak shift from the ideal circulator canbe happened when less uniformity of the ferrite material
property and of the external magnetic field penetrating thedisk, or less symmetric of the stripline geometry, typically.
Later parts of the report explain the situation of the problem and the repair, tests to characterize the circulator, andthe discussion to make them improved.

REPAIR AND THE PERFORMANCE TEST
Problem and the Repair
Discharge in the circulator was happened at the port-1
and the port-2. Problem of the port-1 was found in our facility when problem was happened in end of January 2022,
the problem in the port-2 was found in the factory of themanufacturer. Figure 4 shows the failure of the port-1 because of bad connection between inner stripline and thetransition to the coaxial line. We are guessing that the causeof this failure is thermal cycle of the screws tightened thesetwo components. We don’t know it can be deteriorated fora long-term operation or it is initially in the bad state, but
the event was happened as the discharge in the small gap.
As the temporal solution, we have periodic inspection for
all connection parts if they are still well-tightened.

Figure 5: On-site repair for the port-1 (2022/June).
After that, because of manufacturer strong recommendation, we decided to ship it back to the factory in the US, tohave an inspection by completely disassembling and reoptimizing the magnetic field of permanent magnets. As
shown in Fig. 6, inner stripline was checked and they foundspark traces in the port 2. After detailed check for all components, assembly and magnetic tuning was performed.
The repair activity in the factory was done from July 2022
to March 2023, and the factory acceptance test was performed in February 2023 shown in Fig. 7.

Figure 6: The circulator and the disassembled stripline.

Figure 4: Broken part of the circulator port-1 (2022/Jan).
First action for the repair had been done by polishing theinner stripline and replacing the inner transition to the newone. Inner transition is completely burned and holes for thescrew had been melted. The pair part of the stripline had
also melted, but it was difficult to replace because one large
copper plate is used to the stripline. Our on-site repair result in June 2022 is shown in Fig. 5. Damaged part waspolished, but some dips are still existing. Basically, it is not
a problem, because dip parts are far from the surface. We- 925 -Figure 7: Measurement setup for the factory test.

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP33

In the acceptance test, we noticed that the tuning point
was changed from 25.5 deg. to 30.0 deg., and the existenceof the magnetic materials as the circulator support, but bothinfluences are appeared in the Eq. (4), so we thought itwould compensate by the coil current similar as the ohmic
loss. In the circulator S-parameters test after delivery, werealized that our understanding in the factory test was notcorrect.

VNA Test for the Repaired Circulator
For the repaired circulator, after integration to our RFsystem, we had a test to characterize the circulator performance by S-parameters. Figure 8 shows the setup of themeasurement. To know the effect of near magnetic material,
1cm thickness iron plate was inserted on the bottom of thecirculator. First, we measured the resonance point of Sparameters by scanning the coil current. Next, we checked
the influence of temperature change and the existence ofthe iron plate.

Figure 9: Measurement of the S-parameters for optimized(left) and repaired (right) circulators vs. applied coil current (abscissa).
In addition to that, the peak positions in the right-handside plot looks different. This part is still under study, but
related to this point we had a test by changing temperatureand existence of the magnetic material. Figure 10 shows
the characteristics change for the different temperature andwith and without ~1cm thickness iron plate. Blue, orange,and green plots are, respectively, the normal condition,with inserting an iron plate, and temperature change. 𝑆𝑆21
have similar value, but the effect of temperature makes
peak level changes. In this case, better directivity but worse
return loss was observed. In the Eq. (3) and (4), contributions from temperature appeared in 𝑴𝑴𝟎𝟎 term and from external field 𝑯𝑯𝟎𝟎 are not the same. We are guessing that somepart of differences is coming from there. From the operation point of view, < 26 dB stop band shall be no problem.

Figure 8: Setup of the VNA test for the circulator.
Resonances of each S-parameters for the repaired andoptimized circulator have been performed before highpower test. Frequency response has been checked in thefactory tuning, then the external compensation field was
not applied. Our interesting is the response for the coil current, so we measured all of parameters again also to see theeffect of transportation and construction. The result of peakscan is shown in the plots of Fig. 9. Left hand side denotes
the result of optimized circulator, which is also working as
the pair of the repaired one. Right hand side denotes theresult of the repaired circulator. Peak points of the repaired
one is a bit moved to the right side, which is higher current
region. The reason why this right shift is happening is because of difference of the tuning temperature. The optimized and the repaired circulators are tuned for 25.5 deg.
and 30.0 deg., respectively.

Figure 10: Illustration of the high-power test setup.

The 200kW Full Power Transmission Test to theDummy Load
The RF transmission test for the repaired circulator wasperformed twice in total. The first chance is done just after
onsite repair in June 2022, and the second chance was afterfactory repair in June 2023. In fact, we spent a lot of time
to recover from the circulator failure, and we understandthat many non-effective actions were taken in this series ofactions, but it was good opportunity to start discussion andconsideration of the circulator study as the onsite activity.
We got good result for both tests without any problem inrepaired parts. Especially in the second test, we tested thecirculator performance with existence of the pair circulator.
Th setup was illustrated in Fig. 11. In practical situation,
we polarize each circulator in parallel, and so our interesting working point can be confirmed under interactions of
the magnetic field including its compensation coil inducing
field. As the typical result, the test term for CW-200 kW
transmission was shown in Fig. 12. We successfully- 926 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP33

cleared the test for 200 kW in CW operation for a longterm > 3 hours, which is enough to reach steady state.

the RFQ cavity. We will continue the RF circulation study
by combination of the simulation and the measurement.

Figure 11: Illustration of the high-power test setup.

[1] C. de la Morena et al., “Fully Digital and White Rabbit-Synchronized Low-Level RF System for LIPAc”, in IEEE Transactions on Nuclear Science, vol. 65, no. 1, pp. 514-522, Jan.
2018.
[2] M. Weber et al., “LIPAc RF Power System Engineering Design Report” (Internal).
[3] M. Weber et al., “Functional Overview of the RF Power System for the LIPAc RFQ,” in IEEE Transactions on Plasma
Science, vol. 49, no. 9, pp. 2987-2996, Sept. 2021, doi:
10.1109/TPS.2021.3102840.
[4] T. Shinya et al., “Integration of 175-MHz LIPAc RF Systemand RFQ Linac for Beam Commissioning,” in IEEE Transactions on Plasma Science, vol. 48, no. 6, pp. 1489-1495,
June 2020, doi: 10.1109/TPS.2020.2985020.
[5] Ivan Moya, “LIPAc Master Oscillator and RF integration”(Internal).
[6] H. Bosma, “On the principle of stripline circulation”, The
Institution of Electrical Engineers, pp137-146,1962.
[7] D. K. Linkhart, “Microwave Circulator Design”, ArtechHouse, 2014.
[8] J. Helszajn, “The Stripline Circulator : Theory and Practice”,
John Wiley & Sons, Incorporated, 2008.

REFERENCES

Figure 12: High-power transmission test of the repaired
and its pair circulators for 4-hours in the CW mode.
The return loss of the circulator in the specification sheet
checked in the cold test is more than 26dB, but 22.4dB return loss can be seen in the high-power test. The reason ofthis difference can be imagined because of the thermal effect comes from ohmic loss or the difference of the tuning
point. We still don’t understand about this part, so we muststudy it. Though, in any case, stable reflection could be obtained, and the return level is ~1kW in the absolute value,thus it is not a problem for the operation.

CONCLUSION AND DISCUSSION
Consequently, the circulator had been successfully repaired without whole replacement. Though our couple ofRF test, we also achieved full RF functionality of 200 kW
power transmission in the individual test of the RFQ-RFsystem. Because we are started to use the 200 kW dummyload before this commissioning phase, this is the first trialof the full functional test for the RF system to amplify andtransmit the RF power up to 200 kW.
For the harvest regarding the improvement in the future,we obtained good hints to consider the circulator dynamics.
Asymmetry of the circulator port and the magnetic field
can induce the unbalanced scatter matrix components. As
the issue of the hardware limitation, present current driver,used to the compensation coil has less flexibility for thehigher duty operation. Due to the short stop, circulator temperature can be reduced much lower from the operating
point. So well-designed current control will be required to
the fast recovery.
Considering the difference of the cooling time between
the RFQ and the circulator, it is possible improvement
point to focus on studying about multi-feed RF control of- 927 -